1. Technical Field
The present invention relates to a developing unit for use in an image forming apparatus such as a copying machine, a printer, and the like, and more particularly to improvements of a developing unit using a two-component developer having a magnetic carrier and a toner and an image forming apparatus using the same.
2. Related Art
Generally, development systems used in the electrophotographic process are largely classified into a one-component development system which uses as a developer only a toner constituted by pigment particles and a two-component development system using as the developer one in which magnetic substance particles and a toner constituted by pigmented particles are mixed and agitated.
The one-component development system includes magnetic one-component development using a magnetic toner and a nonmagnetic one-component development using a nonmagnetic toner, and since the magnetic toner does not allow light to transmit therethrough depending on the magnetic powder, the magnetic toner is mainly used as a black toner. Meanwhile, the nonmagnetic toner is suitable for color, but the nonmagnetic toner is currently used principally in low-speed machines in view of the fact that its maintainability is short.
In contrast, since the two-component development system handles the toner with the medium of magnetic substance particles, the two-component development system is able to make use of the magnetically attracting force, handling is facilitated concerning the nonmagnetic toner as well, and it excels particularly in the maintainability. Hence the two-component development system is widely used principally in full-color machines and high-speed machines.
As a conventional developing unit using the two-component development system, one shown in FIG. 33, for example, is generally used.
In the drawing, a developing unit 210 has a development housing 211 with an opening 212 for development formed therein in such a manner as to oppose a latent image carrier 200 such as a photoconductor drum, and development roll 213 serving as a developer carrier is disposed in face-to-face relation to the opening 212 for development, a two-component developer G having a magnetic carrier and a toner is carried on the surface of the development roll 213, and after the thickness of the developer layer is restricted to a predetermined layer thickness by a trimming member 220, an electrostatic latent image on the latent image carrier 200 is converted into a visible image in a development region opposing the latent image carrier 200 of the development roll 213.
Here, the development roll 213 has a development sleeve 214 disposed rotatably so as to rotate, for example, from the upper side toward the lower side in a development region, as well as a magnet member 215 which is fixedly provided inside the development sleeve 214 and in the periphery of which a plurality of magnetic poles 216 are arranged. As the layout of the magnetic poles 216 of the magnet member 215, a development magnetic pole 216 is disposed at a position corresponding to the development region, fixed magnetic poles 216b to 216e are disposed in the order of an S pole, an N pole, an N pole, and an S pole on the downstream side, in the rotating direction of the development sleeve 214, of the development magnetic pole 215a (e.g., N pole). A portion in which the magnetically attracting force is weak or nil is secured midway between the fixed magnetic poles 216c and 216d of the same polarity, and the fixed magnetic pole 216d is disposed before the trimming member 220, thereby allowing the fixed magnetic pole 216d to function as a pickup magnetic pole for capturing the developer.
Further, in this example, a transport paddle 221 which rotates in a direction opposite to the rotating direction of the development roll 213 is disposed at a position adjacent to the development roll 213 inside the development housing 211, e.g., at an opposing portion. Furthermore, a pair of screw augers 222 and 223 for agitating and mixing the developer and circulatingly transporting the developer are disposed on the inner side behind it.
In such a developing unit, the magnetically attracting force acts inside the development roll 213 by the distribution of the magnetic force of the magnet member 215 inside it. The developer G agitated and mixed by the screw augers 222 and 223 is carried and transported on the surface of the development roll 213 by the magnetically attracting force, and after it is restricted to a predetermined layer thickness by the trimming member 220, the developer G reaches the development region, and develops an electrostatic latent image on the latent image carrier 200.
Then, the developer G which lost part of the toner upon completion of development is transported to the downstream side of the development roll 213. However, since a portion in which the magnetically attracting force is weak or nil is formed midway between the fixed magnetic poles 216c and 216d of the same polarity, the transported developer G drops from the surface of the development roll 213 due to the action of the gravity.
Subsequently, the dropped developer G is transported to the screw auger 222 by the transport paddle 221, is further transported in the direction of the axis of rotation of the screw auger 222 and is delivered to the screw auger 223 at an unillustrated delivery section. Even if, for instance, a fresh toner is added there, the added developer G, while being agitated by the screw auger 223, is sent in a direction opposite to the screw auger 222, is delivered again to the screw auger 222 at an opposite delivery section and is transported in the axial direction. At the same time, this developer G is gradually supplied to the vicinity of the development roll 213 by the transport paddle 221.
In this state, the supplied developer adheres to the surface of the development roll 213 by the magnetically attracting force, and the processes in which the developer is subjected to transport, layer formation, and development are repeated again.
Incidentally, in the two-component development system, the attraction between the toner and the carrier making up the developer makes use of the fact that they electrostatically attract each other as a result of the fact that the surfaces of the toner and the carrier respectively charged to mutually opposite polarities by the frictional charging at the time when the toner and the carrier are agitated and mixed. For this reason, it becomes necessary to agitate and mix the developer replenished with the toner, as described above, while the developer is being transported.
In addition, in the development region, it is necessary to control the rate of developer transported within a fixed range so that the developer passes that region with a sufficient amount and does not cause clogging or the like in making contribution to development. For this reason, a method is adopted in which the trimming member is provided to forming the thickness of the developer layer to a predetermined thickness.
Further, in the development region, since the toner in the developer is developed by an electric field formed between the latent image carrier and the developer on the development roll, a development bias voltage is generally applied to the development roll.
The quantity of toner developed at that time is most important among various characteristics that concern the image quality, since it determines the image density. Since the quantity of development toner is determined by neutralizing the electric field in the aforementioned development region by the charge of the toner, it is very important to maintain the charge of the toner to a fixed level.
In the two-component development system, as shown in FIG. 34, the density of the toner in the developer and the amount of toner charge are in an inversely proportional relationship, so that the amount of toner charge can be set to a substantially fixed level by controlling the toner density by adjusting the rate of toner consumption and the rate of replenishing the toner.
[Problems to be Solved]
In the developing unit of this type, as the processes of agitation/mixing, layer formation, and development are repeated in the two-component development system, the developer becomes deteriorated, and the charging capability of the developer declines.
Under such a situation, with the two-component development system which is used for long periods of time, the toner density must be gradually controlled to a low level so as to set the quantity of development toner to a fixed level. However, if the toner density becomes lower than a certain limit, the probability of presence of the toner on the developer surface becomes low, a streaky pattern, which is called a brush mark, occurs on the developed image, thereby possibly exerting an adverse effect on the image quality.
In addition, although the charge of the toner inevitably has a certain degree of distribution, if the amount of toner charged declines with the use of the developer over long periods of time, in the distribution there occur toners which have not been charged or have been charged with an opposite polarity. In this case, the uncharged toner and the toner charged with the opposite polarity are likely to be developed in background portions of the image, and there is hence a possibility of causing so-called fogging.
Furthermore, if the carrier deteriorates with the use of the developer over long periods of time, a situation can occur in which the resistance of the carrier declines. However, when the resistance of the carrier is low to some extent, there is a possibility of the occurrence of so-called carry-over in which the carrier adheres to the latent image carrier side because the electrostatically attracting force between the carrier and the latent image carrier becomes stronger than the magnetically attracting force as the charge is injected from the development roll.
At this time, since the particle size of the carrier is generally quite large and hard as compared with the toner, a faulty close-contact area is formed between the latent image carrier and the recording material which is a sheet to be transferred to is formed in a transfer station. Faulty transfer occurs in this faulty close-contact area, and there is the possibility that blanking with the carrier serving as a nucleus can occur in the transferred image.
Thus, since various problems occur if the developer deteriorates, various techniques have been proposed for minimizing the deterioration of the developer and prolonging the life of the developer.
For example, as a means for reducing the stress in the developer caused by the trimming member, there have been proposed, among others, a method in which the magnetic flux density of the magnetic pole opposing the trimming member is lowered to weaken the magnetically attracting force so as to weaken the pressure applied to the developer (e.g., the Unexamined Japanese Patent Application Publication No. Hei8-278695) and a method in which the trimming member is formed in two stages in such a manner as to sandwich the opposing magnetic pole inside the development roll so as to reduce the stress applied to the developer (e.g., the Unexamined Japanese Patent Application Publication No. Hei10-333431). In each of these proposed methods, however, since a developer pool is generated on the upstream side of the trimming member, and unnecessary pressure is applied to the developer, it has not been successful to prolong the life of the developer substantially.
On the other hand, the life of the developer can be prolonged by increasing the quantity of the carrier used in total, but this results in an increase in cost. Namely, the life of the developer in the two-component development system has been an obstacle in lowering the cost.
Thus, with the two-component developing unit, a major technical problem (a first technical problem) has been how the thickness of the developer layer carried on the developer carrier can be accurately restricted without imparting undue stresses to the developer.
In addition, in, for example, a four-cycle type image forming apparatus, four developing units of yellow, magenta, cyan, and black are disposed around a latent image carrier such as a photoconductor drum, and respective color-component electrostatic latent images on the latent image carrier are developed and made visible and are consecutively multi-transferred onto an intermediate transfer member, thereby forming a color image.
With the four-cycle type image forming apparatus of this type, in a case where the developer is present on the development roll by being in contact with the latent image carrier in the developing units at the time of nondevelopment as well (e.g., developing units of magenta, cyan, and black at the time of the development of a yellow color), since the developer is in contact with the surface of the latent image carrier, there are technical problems in that the developed image on the upstream side may be destroyed by a downstream-side multi-color development brush, and that so-called color mixing can occur in which the toner of another color is mixed in an image of a specific color (e.g., a yellow image).
As prior art for overcoming the technical problems of this type, a proposal has already been proposed in which, for example, the developing units are retracted from the latent image carrier at the time of nondevelopment (e.g., the Examined Japanese Patent Application Publication No. Sho55-3707).
This technique, however, is not desirable since various problems are involved in such as that because the developer is magnetically erect in the development region, the developing units need to move a substantial distance in order to be set in a noncontact state, and it takes time correspondingly, that vibrations occur during retraction, and that the retracting mechanism is complex, making it difficult to render the units compact and lower the cost.
Accordingly, as prior art which does not use such a retracting mechanism, there have been proposed a technique in which the developer on the developer carrier is forcibly scraped and separated by a developer separator (e.g., a blade) (e.g., the Unexamined Japanese Patent Application Publication No. Sho58-34469), as well as a technique in which, in a development roll having a magnet roll disposed inside a rotatable development sleeve, a pair of magnetic poles of the same polarity are formed in an outer periphery of the magnet roll, and during development a magnetic-force declining portion of the pair of magnetic poles of the same polarity is set on the downstream side of the trimming member and on the upstream side of the development region, while during nondevelopment the magnetic-force declining portion is set at a portion opposing the trimming member, so as to block the transport of the developer to the downstream side of the trimming member during nondevelopment (e.g., the Unexamined Japanese Patent Application Publication No. Hei5-289520).
However, in the case where the developer separators of the above-described prior art are used, large stresses are applied to the developer at the time of separation of the developer, so that it cannot be said that such a technique is desirable in terms of the prolongation of the life of the developer.
In addition, in the case of the latter prior art as well, a developer pool is generated on the upstream side of the trimming member during both development and nondevelopment, and exerts unnecessary pressure to the developer, so that this technique cannot be said to be preferable in terms of the prolongation of the developer.
Meanwhile, as other prior art which does not use the retracting mechanism and does not apply large stresses to the developer, the following techniques are known, for example: A technique in which the supply to the developer carrier is stopped by stopping the rotation of an auxiliary fixed magnet for supplying the developer to the developer carrier (e.g., the Unexamined Japanese Patent Application Publication No. Sho63-229466); a technique in which, in a development roll having a fixed magnet roll (provided with a pair of magnetic poles of the same polarity among its magnetic poles) disposed inside a rotatable development sleeve, the developer on the development roll is caused to drop at the point of time when it has passed the pair of magnetic poles of the same polarity of the magnet roll by reversely rotating the development sleeve at the time of nondevelopment, to thereby prevent the developer from being held on the development roll in the development region (e.g., the Examined Japanese Patent Application Publication No. Hei6-25880); and a technique in which the positions of magnetic poles are changed by rotating the magnet roll inside the development sleeve at the time of nondevelopment, and a portion between the pair of adjacent magnetic poles is disposed in such a manner as to oppose the latent image carrier, so as to suppress so-called color mixing (e.g., the Unexamined Japanese Patent Application Publication No. Hei5-289491).
However, with the above-described first prior art (the Unexamined Japanese Patent Application Publication No. Sho63-229466), even if the developer is not supplied to the developer carrier by the auxiliary fixed magnet, if the developer carrier rotates, there is a possibility of the developer becoming carried by the distribution of the magnetic forces on the developer carrier side, so that it is difficult to completely avoid color mixing.
Further, with the above-described second prior art (the Examined Japanese Patent Application Publication No. Hei6-25880), in the case where a plurality of developing units are rotated by one driving motor, it is inevitable to additionally install a large number of components such as clutches and gears, so that the configuration of the drive system becomes wastefully complex.
Furthermore, with the above-described third prior art (the Unexamined Japanese Patent Application Publication No. Hei5-289491), since the developer layer on the developer carrier in the development region is merely in a state in which its spikes are laid, and not in a state in which the developer layer is completely removed, it is difficult to completely prevent the so-called color mixing in a case where the gap between the developer carrier and the latent image carrier is small.
Thus, with, for example, the four-cycle type image forming apparatus, a major technical problem (a second technical problem) has been how the amount of developer carried on the developer carrier is adjusted to zero without imparting undue stresses to the developer during nondevelopment, i.e., how adjustment is to be made into a state in which the developer is not supplied to the development region.
It should be noted that such a technical problem similarly occurs with a type in which a plurality of color-component toner images are formed on a common latent image carrier as with a tandem two-cycle type image forming apparatus.
The invention has been devised to overcome the above-described first technical problem, and its object is to provide a developing unit capable of accurately restricting the thickness of the developer layer carried on the developer carrier without imparting undue stresses to the developer at the time of development, as well as an image forming apparatus using the same.
In addition, the invention has been devised to overcome the above-described second technical problem, and its another object is to provide a developing unit capable of restricting to substantially zero the thickness of the developer layer carried on the developer carrier without imparting undue stresses to the developer at the time of nondevelopment, as well as an image forming apparatus using the same.
[Means for Solving the Problems]
Namely, to overcome the first technical problem (restriction of the thickness of the developer layer at the time of development), in accordance with the invention, as shown in FIG. 1, there is provided a developing unit which has a development housing 2 with an opening 2a for development formed therein in such a manner as to oppose a latent image carrier 1, and in which a developer carrier 3 is disposed in face-to-face relation to the opening 2a for development, and a two-component developer G having a magnetic carrier and a toner is carried on a surface of the developer carrier 3, so as to convert an electrostatic latent image on the latent image carrier 1 into a visible image in a development region m opposing the latent image carrier 1 of the developer carrier 3, characterized in that the developer carrier 3 is provided with a development sleeve 4 for carrying the two-component developer G and disposed rotatably so as to rotate from an upper side toward a lower side in the development region m as well as a magnet member 5 which is fixedly provided inside the development sleeve 4 and in a periphery of which a plurality of magnetic poles 6 are arranged, and that the magnet member 5 is provided with a development magnetic pole 6a disposed at a position corresponding to the development region m and contributing to development, a pickup magnetic pole 6b disposed in a region which is located on a lower side than the height of a rotational center O of the development sleeve 4 in a region where the development sleeve 4 in terms of its rotating direction is headed from the lower side toward the upper side, and adapted to capture the developer G inside the development housing 2 onto a surface of the development sleeve 4, and a trimming magnetic pole 6c disposed on a downstream side, in the rotating direction of the development sleeve 4, of the pickup magnetic pole 6b and adapted to restrict a developer layer into a predetermined thickness by causing an excess portion of the developer captured by the pickup magnetic pole 6b to drop in a vicinity of the height of the rotational center O of the development sleeve 4 and thereby causing only a necessary amount of the developer G to be carried.
In the above-described technical means, the developer carrier 3 may be selected as required insofar as it is provided with the rotatable development sleeve 4 which rotates from the upper side toward the lower side in the development region m, and the magnet member 5 is fixedly provided inside the development sleeve 4.
In such a case, the magnetic member 5 may be selected as required insofar as it is provided with the plurality of magnetic poles 6. However, to allow the magnet forces from the magnetic poles 6 to uniformly act on the surface of the development sleeve 4, it is preferable to provide the magnetic poles 6 in outer peripheral portions of the roll-like member or to adopt a technique in which the outer peripheral portions of the roll-like member are magnetized to predetermined magnetic forces by an unillustrated magnetizer.
In addition, of the magnetic poles 6 of the magnet member 5, the development magnetic pole 6a may be of a single-piece configuration or a multiple-piece configuration insofar as it contributes to development. In the mode of the multiple-piece configuration, an appropriate arrangement may be selected as required. For example, one of the development magnetic poles 6a maybe disposed in such a manner as to oppose a central portion of the development region m, or may be opposingly disposed at a position offset from the center of the development region m. Alternatively, portions between adjacent ones of the plurality of magnetic poles may be disposed in such a manner as to oppose the central portion of the development region m.
Further, the pickup magnetic pole 6b may be of a single-piece configuration or a multiple-piece configuration insofar as it has the function of being capable of capturing the developer G on the lower side than the height of the rotational center O of the development sleeve 4. However, it is necessary for the amount of development captured to be greater than the required amount carried on the developer carrier 3.
Furthermore, the trimming magnetic pole 6c may be either of a single-piece configuration or of a multiple-piece configuration insofar as it is disposed on the downstream side, in the rotating direction of the development sleeve 4, of the pickup magnetic pole 6b, and exhibits the action of magnetic force for restricting only a necessary amount of developer in the vicinity of the height of the rotational center O of the development sleeve 4.
Here, a description will be given of the preferred forms of the invention. It is preferred that the trimming magnetic pole 6c be set to have a weaker magnetic field than the pickup magnetic pole 6b in view of the ease of formation of the magnetic force distribution whereby an excess portion of the developer G captured by the pickup magnetic pole 6b is caused to drop reliably in the vicinity of the height of the rotational center O of the development sleeve 4.
Nevertheless, even if the trimming magnetic pole 6c has a stronger magnetic field than the pickup magnetic pole 6b, if a mode is adopted in which the trimming magnetic pole 6c is sufficiently spaced apart from the height of the rotational center O of the development sleeve 4, it is possible to appropriately adjust the magnetic field distribution in the vicinity of the height of the rotational center O of the development sleeve 4.
In addition, although the layout of the trimming magnetic pole 6c may be selected as required, from the viewpoint of the ease with which the trimming action (the action of restricting the developer layer thickness) of the trimming magnetic pole 6c can be demonstrated, the trimming magnetic pole 6c should preferably be disposed at a position higher than the height of the rotational center O of the development sleeve 4.
Further, to reliably transport to the development region m the developer G with its layer thickness restricted by the trimming magnetic pole 6c, the magnet member 5 may preferably have a transporting magnetic pole 6d disposed between the trimming magnetic pole 6c and the development magnetic pole 6a in the rotating direction of the development sleeve 4 and adapted to transport the developer with its layer thickness restricted by the trimming magnetic pole 6c. 
Furthermore, in a case where it is desirable to temporarily separate the developer G on the developer carrier 3 before capturing the developer G by the pickup magnetic pole 6b, a pair of magnetic poles of the same polarity (not shown) for separating the developer G may be provided on the downstream side, in the rotating direction of the development sleeve 4, of the development region m. In this case, the downstream-side magnetic pole of the pair of magnetic poles of the same polarity may be used as the pickup magnetic pole 6a. 
In addition, in the invention, it goes without saying that the developing unit may be provided with various measures.
For example, as a mode which makes it possible to increase the efficiency of replacing the developer G, it is possible to cite a mode in which a dropping assisting mechanism is provided for causing an excess portion of the developer G captured on the surface of the development sleeve 4 by the pickup magnetic pole 6b to drop on the lower side than the height of the rotational center O of the development sleeve 4.
In this mode, the developer G which was deprived of its toner can be temporarily moved away from the developer carrier 3 to be replenished with the toner, and the developer G replenished with the toner can be supplied again to the developer carrier 3. Therefore, this mode is effective in continuously performing development at a high area ratio.
Further, in this embodiment, since the developing unit is arranged that the layer thickness of the developer G on the developer carrier 3 is restricted by the trimming magnetic pole 6c, it is unnecessary to separately provide a layer-thickness restricting member (trimming member) at a position opposing the developer carrier 3.
However, from the viewpoint of avoiding the leakage of the developer from the development housing 2 as much as possible, a sealing member 7 for sealing a gap between the development housing 2 and the developer G layer should preferably be provided over a periphery of the developer carrier 3 on which the developer G with its layer thickness restricted by the trimming magnetic pole 6c is transported to the development region m.
This sealing member 7 may be selected as required by such as forming an edge of the opening 2 for development in the development housing 2 into a preferred shape, or separately providing the sealing member inside the development housing 2.
The sealing member 7 referred to herein means not a trimming member but one which at least does not cause the developer G to drop from the developer carrier 3 by blocking the developer G. Further, in the mode in which the trimming member also serves as a seal on the upstream side of the development region as in a conventional developing unit, at an outlet of the developer where the developer is blocked by the trimming member and a layer is formed, a toner cloud is relatively prone to occur since the developer which was subjected to the force is released. In the invention, however, since a large force is not applied to the developer G, the amount of toner cloud generated at the output of the sealing member 7 is suppressed.
Furthermore, to accurately restrict the layer thickness of the developer G on the developer carrier 3, as its precondition it becomes necessary to supply a sufficient amount of developer G to the pickup magnetic pole 6b of the developer carrier 3.
Accordingly, as an effective mode in supplying a sufficient amount of developer G to the pickup magnetic pole 6b, it is possible to cite a mode in which a developer supplying member (not shown) for supplying the developer G is disposed inside the development housing 2 at a portion of the developer carrier 3 corresponding to the pickup magnetic pole 6b. 
Additionally, from the viewpoint of satisfactorily maintaining the charging properties of the developer, an agitating member (notshown) for agitating the developer G should preferably be disposed in the development housing 2.
In addition, the layout of the magnetic poles 6 of the magnet member 5 may be selected as required insofar as the aforementioned development magnetic pole 6a, pickup magnetic pole 6b, and trimming magnetic pole 6c are included. However, from the viewpoint of enhancing the degree of freedom of locating the position where the development region m is provided, the magnet member 5 is preferably one in which magnetic poles 6 of different polarities are arranged alternately.
According to this mode, the degree of freedom in the layout of the position where the development region m is provided can be enhanced by the portion in which the pair of magnetic poles of the same polarity need not be provided on the downstream side of the development region m.
In addition, from the viewpoint of making the developing unit compact, only the developer carrier 3 should preferably be disposed in the development housing 2.
Further, to satisfactorily maintain the circulating properties of the developer G inside the development housing 2, the development housing 2 should preferably have a guide portion (not shown) for guiding the developer G dropping from the developer carrier 3 toward a developer storage section.
This guide portion includes, for instance, an inclined portion which allows the developer G to move toward the developer storage section by its own weight.
It should be noted that although the developer storage section refers to a portion where the developer G is stored, it goes without saying that an agitating member for the developer G, a developer supplying member, and the like may be provided.
In addition, as described above, the developing unit in accordance with the invention is so arranged that the layer thickness of the developer G on the developer carrier 3 is restricted by the trimming magnetic pole 6c, so that it becomes unnecessary to separately provide a layer-thickness restricting member (trimming member) at a position opposing the developer carrier 3. However, to maintain more satisfactorily the state of the developer G layer being transported to the development region m, a leveling member 8 for leveling the developer G layer should preferably be provided over a periphery of the developer carrier 3 on which the developer G with this layer thickness restricted by the trimming magnetic pole 6c is transported to the development region m.
Here, the leveling member 8 is sufficient if it has the function of leveling the developer G layer, and the leveling member 8 includes one which blocks a portion of the developer. Nevertheless, to reliably suppress the stresses to the developer G, it is preferable to eliminate the function of blocking the developer G.
Further, in the invention, an agitating member such as a screw auger for agitating the developer G is, to be sure, effective in enhancing the charging properties of the developer G. On the other hand, there is a drawback in that the life of the developer G is shortened since the developer G is forcibly agitated. For example, in the case where the screw auger is used, it is absolutely necessary to dispose two screw augers in two directions so that the developer will not be offset. However, in this returning section, since the direction is changed by applying pressure to the group of developers, the life of the developer is shortened. If an attempt is made to make this arrangement smooth, it leads to a large-size unit and higher cost.
Meanwhile, since a conventional trimming member need not be disposed over the periphery of the developer carrier 3, it is possible to make use of the space for installation of this trimming member.
Accordingly, from the viewpoint of minimizing the agitating member for the developer G or of disusing it, a charging member 9 for charging the developer G should preferably be provided over a periphery of the developer carrier 3 on which the developer G with this layer thickness restricted by the trimming magnetic pole 6c is transported to the development region m.
Here, as a preferable form of the charging member 9, it is possible to cite one in which the charging member 9 is formed by a charging projecting member for charging the developer G without causing the developer G to drop in a direction opposite to the rotating direction of the developer carrier 3.
This charging projecting member which is the charging member 9 is disposed continuously or discontinuously along the rotating direction of the developer carrier 3, charges the layer of the passing developer G while imparting turbulence thereto, and is preferred because the charging efficiency is high.
In addition, the charging member 9 suffices if it exhibits charging action based on friction with the layer of the developer G which passes, but a charging electric field may be caused to act in the charging member 9.
As a preferred form of the charging electric field at this time, it suffices if an alternating electric field is formed in a region where the charging member 9 and the developer carrier 3 are opposed to each other.
In addition, in the mode in which the charging member 9 is provided over the periphery of the developer carrier 3, since the surface of the developer G layer is somewhat disturbed when the developer G layer passes through the charging member 9, the leveling member 8 for leveling the developer G layer should preferably be provided on the downstream side, in the rotating direction of the developer carrier 3, of the charging member 9.
However, if the magnetic pole 6 such as the transporting magnetic pole 6d is provided on the downstream side, in the rotating direction of the developer carrier 3, of the charging member 9 before the development region m, the developer G layer, when passing by the transporting magnetic pole 6d, is leveled to some extent in accordance with the distribution of magnetic force from the transporting magnetic pole 6d, the leveling member 8 is not essentially required.
Next, by way of comparison it will be described that a developing unit in accordance with a comparative model shown in FIG. 33 (having a trimming member 220) is not preferable for the life of the developer, and that the developing unit in accordance with the invention shown in FIG. 1 is preferable for the life of the developer.
According to studies conducted by the present inventors, a decline in the charging capability of the developer, which constitutes a major cause of the decline in the life of the developer, is due to a decline in the charging capability of the carrier.
Namely, the present inventors confirmed that the effective charging surface area of the carrier decreases as toner-side components, i.e., a binder for the toner, external additives used on the toner surfaces for the purpose of charging control and securing fluidity, and the like, become fixed to the carrier surfaces.
In addition, the cause of occurrence of the above-described fixation of the toner-side components to the carrier surfaces is that the toner and the carrier are coupled to each other through frictional charging. When the degree of fixation of the toner-side components to the carrier surfaces was examined by shutting off the transfer of the developer between the respective sections and by independently moving the developer in order to examine in which section of the developing unit the effect is strong, it was found that the formation of the layer by the trimming member contributes most greatly.
The pressure in the section for forming the layer by using the trimming member 220 was considered as shown in FIG. 35.
To cut off the spikes smoothly with respect to the developer G layer on a development roll 213, the trimming member 220 is disposed slightly downstream of a fixed magnetic pole (e.g., 216d). When the developer G is blocked by the trimming member 220 and the state is set in a steady state, a large amount of developer G is held on the upstream side of the trimming member 220 by the fixed magnetic pole 216d. From this large quantity of the group of developer G, part of the developer G on a development sleeve 214 side passes in the form of a layer through the gap between the trimming member 220 and the development roll 213.
At this time, at a boundary portion (indicated by a dotted line in FIG. 35) between the developer G which slips through and the developer G which remains, a magnetically attracting force F acts due to the magnetization of the developer G located outwardly of the boundary portion. Namely, the greater the amount of developer G held, the stronger this magnetically attracting force F. Further, the force obtained by multiplying that magnetically attracting force F by the coefficient of friction at the boundary portion becomes a transporting force S (frictional force) at the boundary portion. Accordingly, the greater the amount of developer G held on the upstream side of the trimming member 220, the greater the transporting force S and the greater the area of the boundary portion where the frictional force S acts. Further, the group of developer G is pressed by the trimming member 220 and is thereby subjected to large pressure.
It is conceivable that the deterioration of the developer G is promoted by the accumulation of the large quantity of developer G on the upstream side of the trimming member 220 in the above-described manner.
Here, although the accumulation of the large quantity of developer G can be prevented by setting to a small level the magnetic flux density of the fixed magnetic pole 216d on the upstream side of the trimming member 220, the situation in which the layer forming operation is made unstable is unavoidable.
Namely, since the developer G supplied to the development roll 213 is subjected to the effect of, for instance, the pitches of blades of a transport paddle 221 and screw augers 222 and 223 (see FIG. 33) for supplying the developer G, if the amount of accumulation of the developer G layer on the upstream side of the trimming member 220 is small, the effect directly appears in the developer G layer and, hence, in the image.
For this reason, it can be understood that the accumulation of the developer G before the trimming member 220 contributes to the stabilization of the layer forming operation.
Accordingly, in the invention, in view of these circumstances, a method for effecting the layer formation without using the trimming member 220 was newly devised.
Namely, in the invention, as shown in FIG. 3, the layout of the magnetic poles 6 of the magnet member 5 is so devised that after the developer G is captured at a portion of the developer carrier 3 corresponding to the pickup magnetic pole 6b, the excess developer G which cannot be held on the developer carrier 3 by the magnetically attracting force is allowed to drop in the section leading to the trimming magnetic pole 6c so as to hold a necessary amount of developer G.
In accordance with this mode, the fixation of the toner component to the carrier can be suppressed as much as possible since there is no portion where the developer G slips through with friction and a large quantity of developer G attached to the fixed magnetic pole 216d (a strong magnetic field portion) is not pressed strongly against the trimming member 220 as is the case with the comparative model using the trimming member 220 for forming the layer.
Here, if detailed consideration is given to the process of transporting the developer G with reference to FIG. 3, it is considered that the force for transporting the developer mainly consists of the magnetically attracting force of the developer carrier 3, the coefficient of friction between the developer carrier 3 and the developer G, the coefficient of friction between the particles of the developer G, gravity, and inertial force. Of these elements, description of the magnetically attracting force and the inertial force will be omitted for the sake of simplicity because the magnetically attracting force in the tangential direction of the developer carrier 3 is complicated since it differs depending on the positional relationship between the developer G and the magnetic poles 6, because the inertial force is complicated since it differs depending on the rotating speed and diameter of the developer carrier 3, and because no discrepancy arises even if these elements are ignored in a general explanation in comparison with an actual phenomenon.
Now, on the surface of the developer carrier 3 corresponding to the pickup magnetic pole 6b, the gravity and a vertical component (shown by a thin line in the drawing) of the magnetically attracting force acting in the normal direction are balanced on the surface of the group of the developer G, as shown in a portion A in FIG. 3.
Subsequently, when the developer G held in the vicinity of the surface of the developer carrier 3 is transported to a height (a portion B in FIG. 3) substantially identical to the position of the rotational center O of the developer carrier 3 from the portion of the pickup magnetic pole 6b (a portion Axe2x80x2 in FIG. 3), the magnetically attracting force is considerably weak.
At this time, the forces for upwardly transporting the developer G are the magnetically attracting force and the frictional force based on the coefficient of friction, and since the magnetically attracting force has become weak, the position where this frictional force and the gravity are balanced is located on the inner side of the developer G layer as compared with the position on the pickup magnetic pole 6b. Consequently, the developer G located outwardly thereof drops from the developer carrier 3 (the portion B in FIG. 3).
As a result, the developer G is restricted to a predetermined layer thickness in the vicinity of the height of the rotational center O of the developer carrier 3.
When the developer G is further transported upwardly, as shown in a portion C in FIG. 3, the component of the gravity normal to the developer carrier 3 becomes oriented toward the developer carrier 3, so that the force acting in the tangential direction of the developer carrier 3 (the force for transporting the developer G), which is obtained by multiplying the sum of this force and the magnetically attracting force by the coefficient of friction of the developer G, increases.
At this time, if the force for transporting the developer G is lower than the tangential component of the gravity, the developer G drops, and if it is not lower, the developer G does not drop.
Here, in order for the transporting the developer G to be lower than the tangential component of the gravity, it is necessary that the magnetically attracting force has dropped more than the component of the gravity normal to the developer carrier 3 has increased in comparison with the portion B in FIG. 3. Unless such a situation occurs, the developer G is transported to the development region m as it is with the predetermined layer thickness without dropping from the developer carrier 3.
Accordingly, in order to form the developer G into a thin layer of a predetermined thickness on the developer carrier 3 in the section leading from the pickup magnetic pole 6b to the trimming magnetic pole 6c, it is preferable to make the design so that the excess developer G drops in the vicinity of the height of the rotational center O of the developer carrier 3. For this purpose, it is necessary for the pickup magnetic pole 6b to be located on the lower side than the position of the rotational center O of the developer carrier 3.
In addition, major factors for determining this condition of layer formation are the intensities of and the positional relationship between the magnetic poles 6, particularly the pickup magnetic pole 6b and the trimming magnetic pole 6c. Other factors contributing to the condition of the layer formation include, among others, the surface roughness of the developer carrier 3, the coefficient of internal friction or the fluidity of the developer G, the diameter and rotating speed of the developer carrier 3. To obtain a more desirable developer G layer, it is necessary to make adjustment with respect to these factors as well.
It should be noted that although, of the applications filed by the present applicant, a technique similar to the invention of this case has been filed as an earlier application, it is supplementarily stated here that that technique is different in the concept from this case.
Namely, the above-described earlier application (the Japanese Patent Application No. Hei11-281199) is characterized in that xe2x80x9cthe magnetic force and magnetic force distribution of the magnetic-field generating means in the pickup region and the transport region are set such that a difference between the amount of developer picked up onto the development roll in the pickup region and the amount of developer dropping from the development roll in the transport region constitutes a necessary amount of developer required in the development region.xe2x80x9d
This earlier application is premised on the fact that the respective functions (pickup, transport, and development) in the respective regions are demonstrated by setting the magnetic force and the magnetic force distribution in each region (pickup region, transport region, and development region). In addition, the amount of developer dropping in the transport region mainly refers to the developer which drops when it is leveled by a leveling member. Therefore, this earlier application differs in the concept from the characteristic feature of the invention of this case: xe2x80x9ca trimming magnetic pole disposed on a downstream side, in the rotating direction of the development sleeve, of the pickup magnetic pole and adapted to restrict a developer layer into a predetermined thickness by causing an excess portion of the developer captured by the pickup magnetic pole to drop in a vicinity of the height of the rotational center of the development sleeve and thereby causing only a necessary amount of the developer to be carried.xe2x80x9d
In addition, to overcome the above-described second technical problem (restriction of the thickness of the developer layer at the time of nondevelopment), in accordance with the invention, it suffices if, as shown in FIG. 2, there is provided a developing unit which has a development housing 2 with an opening 2a for development formed therein in such a manner as to oppose a latent image carrier 1, and in which a developer carrier 3 is disposed in face-to-face relation to the opening 2a for development, and a two-component developer G having a magnetic carrier and a toner is carried on a surface of the developer carrier 3, so as to convert an electrostatic latent image on the latent image carrier 1 into a visible image in a development region m opposing the latent image carrier 1 of the developer carrier 3, characterized in that the developer carrier 3 is provided with a development sleeve 4 for carrying the two-component developer G and disposed rotatably so as to rotate from an upper side toward a lower side in the development region m as well as a magnet member 5 which is fixedly provided inside the development sleeve 4 and in a periphery of which a plurality of magnetic poles 6 are arranged, and that the magnet member 5 is so arranged that the layout of the magnetic poles is set variably between the time of development and the time of nondevelopment, and the amount of developer transported to the development region m while being carried on the surface of the developer carrier 3 is set to substantially zero at the time of nondevelopment.
In such a technical means, the layout of the magnetic poles of the magnet member 5 at the time of development is not particularly limited, and an appropriate layout may be selected insofar as, at the time of nondevelopment, the layout of the magnetic poles changes to one in which the amount of development carried on the surface of the developer carrier 3 and transported to the development region m is set to substantially zero.
In addition, invariably setting the layout of the magnetic poles between the time of development and the time of nondevelopment, the entire magnet member 5 may be moved, or some magnetic poles 6 may be moved.
Further, as the layout of the magnetic poles at the time of nondevelopment, a mode may be adopted in which the amount itself of developer G picked up onto the developer carrier 3 is set to substantially zero, or a mode may be alternatively adopted in which the developer g is temporarily picked up onto the developer carrier 3, but all the developer G picked up is caused to drop.
Here, as the former layout of the magnetic poles at the time of nondevelopment, in a mode in which, for example, the magnet member 5 is provided with repellent magnetic poles (a pair of magnetic poles of the same polarity) for separating the remaining developer, it is possible to cite a mode in which the pair of magnetic poles of the same polarity are moved to a position corresponding to the pickup region for picking up onto the developer carrier 3, and the amount itself of developer G picked up onto the developer carrier 3 is set to substantially zero.
In addition, as the latter layout of the magnetic poles at the time of nondevelopment, in the case where the layout of the magnetic poles at the time of development is the same as the mode shown in FIG. 1 (development magnetic pole 6a, pickup magnetic pole 6b, and trimming magnetic pole 6c), it is possible to cite a mode in which, as shown in FIG. 2, by making use of the fact that the amount of the layer thickness of the developer G restricted differs due to the positional change of at least the trimming magnetic pole 6c, the amount of the layer thickness of the developer G restricted is set to substantially zero by moving the position of the trimming magnetic pole 6c. 
In particular, the latter example of the layout of the magnetic poles at the time of nondevelopment is preferable in that, by variably setting the layout of the magnetic poles, the developer with its layer thickness restricted can be supplied to the developer carrier 3 at the time of development, and the supply of the developer can be stopped at the time of nondevelopment.
In addition, in the mode in which the position of at least the trimming magnetic pole 6c is variably set, it suffices if, at the time of nondevelopment, the trimming magnetic pole 6c is displaced more on the upstream side in the rotating direction of the development sleeve 4 than the position used at the time of development.
In this case, since the range of change of the trimming magnetic pole 6c can be controlled to a relatively small range, this mode is preferable in that the layout of the magnetic poles can be controlled to a minimum.
In addition, although the invention of the modes shown in FIGS. 1 and 2 has the developing unit as its subject, the invention is not limited to the same, and may have as its subject an image forming apparatus itself which uses the developing unit.